Microheterogeneity of the growth-associated neuronal protein B-50 (GAP-43) : Contribution of phosphorylation by protein kinase C

The neuron-specific, growth-associated protein B-50, also known as GAP-43, F1 and neuromodulin, shows a striking heterogeneous behaviour in many chromatographic and electrophoretic systems. A modulatory function has been proposed for the protein in receptor-mediated processes in the presynaptic membrane. Fatty acid acylation, calmodulin binding and phosphorylation appear to be tools in this respect. At least three discrete isoforms were present in separations made by reversed-phase fast protein liquid chromatography (FPLC) of the phosphorylated protein. In anion-exchange FPLC chromatography a conglomerate of eight peaks was eluted, which migrated as eight parallel curves in electrophoretic mobility studies. After dephosphorylation of the protein this number was reduced to two. Under non-reducing conditions, the phosphoprotein was eluted from an FPLC gel filtration column at Mr = 270 kDa, i.e. 8–12 times the size of the monomer (m = 23.6 kDa). In sodium dodecyl sulphate polyacrylamide gel electrophoresis all isoforms showed only B-50 at Mr of 48 kDa and its breakdown product (Mr = 40 kDa) in a constant ratio. It was concluded that phosphorylation by protein kinase C of a single serine residue is only one factor in the microheterogeneity of B-50. Multimeric forms may also add to the heterogeneous behaviour of phosphorylated B-50.